Following the instructions at https://www.dropbox.com/install-linux you can install it in a Linux Terminal.
However, Dropbox recently made a very sad decision to not support Linux (except ext4 unencrypted, which is a nonstarter), so Dropbox is no longer possible with CoCalc.

First, despite the fact that you are accessing CoCalc through the internet,
you are actually working in a highly restricted environment.
Processes running inside a free project are not allowed to directly access the internet.
(We do not allow such access for free users, since when we did,
malicious users launched attacks on other computers from CoCalc.)
Enable internet access by adding the “internet access” quota. See Project Upgrades.

Once you have enabled network access for your project,
to use ssh from CoCalc to connect to GitHub
you need to create an SSH key and explicitly allow that account/key access to GitHub.

to start the website as an “app”.
(If you use chromium instead, type chromium-browser--app=https://cocalc.com/app).

This is not a completely solution since in some cases,
control-shift-minus and control-shift-plus still zoom in and out
(on ChromeOS they zoom the entire desktop – every window, all icons, the time, etc., in and out!).

This is a major problem, because control-shift-minus is “undo” in emacs.
The workaround I currently use is to put this in my .emacs file,
and instead type “alt-u” for undo:

(define-keyesc-map"u"'undo)

Note that undo is also available by default through other key bindings,
like C-x u and C-/. (Execute “M-x where-is undo”.)
So there are other options available.

The Pentadactyl plugin is not compatible with the Codemirror editor in CoCalc.
You can disable Pentadactyl for a particular page, perhaps by pressing Control+Z.
CoCalc has it’s own Vim bindings for worksheets and editing files;
to enable it, go to Project Settings (click the gear by your name in the upper right), then select Vim next to Editor → Bindings.